• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

NAD 7020 needs help finding L channel distortion.

tusntuk2

tusntuk2

Active Member
I am trying to find the cause of serious distortion in one channel of a NAD 7020. The pre amp sounds clean, but the L side of the power amp has issues. Gets worse with more bass or more volume. I haven't done any more than try to de-ox and lube pots and switches so far. This is an old model, everything is crammed onto one pcb, and it's a mess. Looks like someone's been in here before, too, which adds to the dilemma, with a couple of things soldered underneath, but it also looks like original caps, etc. Help! From what I hear on the right side the amp is worth saving, but I'm having trouble tracing circuits with limited tools. Any ideas?
 
Register to hide this ad
Since only one channel affected I would be leaning towards the power amp rather than the power supply.
NAD of that era had 2 (actually a few more) issues,
- cheap and nasty caps
- dry solder joints (google has pix)

Visually inspect caps, I'd be tempted to replace any signal path caps on the right channel
eg, C602,4. Also have a close look for dry/bad solder joints
 
Not another defect. Add cheap trimpots to the list. A good cleaning and reset of bias and dc offset could clear the problem. Better to replace them.
 
Many thanks, gentlemen! I did isolate the power amp to determine the distortion was caused there. No visual evidence of cap leakage, but I know that is inconclusive too. And sooo much brown glue! YUK. Replace trim pots? Hmmm, haven't tried to cross reference those before - I will try to get some bias voltage measurements if I can. Isn't this the one where I short out a resistor? I'm not so familiar with setting the bias on this amp. dc offset read 27mv and 11mv. And as I said before, someone did some work on this little guy before, there are some nasty solder blobs and more than one jumper wire that I bet is not original. Anybody have diagrams or pics of the big pcb? Again, thank you so much for your generous help!
 
OK, found the manual, it says parallel R647 or 648 with a 1K, so I'll try to find one in the box. Here is a curiousity to me - although the distortion is plainly evident in the L channel speaker, it is absent in the headphone jack. Why might that be? Aren't they using the same output source? At least that gives me hope, and the headphones sound wonderful.
 
headphones require less current = easy to drive.
Something is open around the driver/ output transistors, although the bias current reading suggests otherwise.
 
Nakdoc said:
headphones require less current = easy to drive.
Click to expand...
Maybe you could try driving the speakers at very low volumes and not any distortion.
In any case I'd give the speaker selector switch a liberal spray of deoxit etc...
Maybe set speaker selector to A+B, distortion?
Also you could try connecting the speakers to B and see if distortion is present
 
Does this model have an output relay as bad contacts could also be the issue with a bad speaker output - if the headphone socket is fed from the speaker outputs as well, that is?
 
You must log in or register to reply here.
Back
Top Bottom